(1) Field of the Invention
The present invention relates generally to the manufacture of polymer materials, and more particularly to a method for increasing fracture toughness and reducing brittleness of a semi-crystalline polymer material such as poly(vinylidene fluoride-trifluorethylene) or p(VDF-TrFE).
(2) Description of the Prior Art
Many semi-crystalline polymers become brittle when formed into thin sheets. In terms of a quantitative measure, these materials have a low fracture toughness which is measured as energy per unit volume in Joules/meter3 (J/m3). Brittleness is caused by a high percentage of crystallinity and/or an increased average size of the polymer crystallites brought about by the manufacturing process. As a result of the material""s brittleness, damage during normal handling thereof is prevalent thereby increasing the cost of using semi-crystalline polymers in various products.
In some applications, crystallinity percentages in excess of 80% are desired or required in order for the semi-crystalline material to perform properly. For example, in order to optimize the performance of certain electroactive polymers, it is necessary to anneal the material to a very high level of crystallinity. However, while the annealing step greatly increases the material""s crystallinity in preparation for a ferroelectric poling operation, this processing step also makes the treated material so brittle that it often cracks during routine handling thereof.
Accordingly, it is an object of the present invention to provide a method for increasing a semi-crystalline polymer""s fracture toughness to thereby reduce its brittleness.
Another object of the present invention is to provide a method that reduces the costs associated with using brittle semicrystalline polymer materials.
A still further object of the present invention is to provide for increased use of semi-crystalline polymer materials in applications where the material""s brittleness previously prevented such use.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method is provided for increasing fracture toughness and reducing brittleness of a semi-crystalline polymer material. The material is placed in an inert oxygen-free atmosphere and heated to a temperature that is greater than room temperature but below the melting temperature of the material. The material is then irradiated with beta particles until a desired level of fracture toughness is achieved. Fracture toughness is a function of the radiation dose received by the material. Specific processing steps are provided for the semi-crystalline poly(vinylidene fluoride-trifluorethylene) or p(VDF-TrFE).